Lead battery performance has steadily improved over the last 100 years with incremental developments accelerating that improvement in the last two decades. Here, Francisco Trinidad, PhD Electro-chemistry, and Independent Advisor, gives BEST an overview of his talk on the latest versions of the lead-acid battery being developed to cope with ever changing requirements.
The electrification of the automotive sector and, more recently, the increasing energy storage demand are the main drivers for battery innovation. Both applications require maintenance free operation, high specific power in charge and discharge and moderate volumetric energy density.
After the invention of valve regulated lead-acid (VRLA) battery, initially through acid jellification (GEL) and later by glass mat absorption (AGM), the internal gas recombination reaction allowed full maintenance free operation. However, it was in this century that VRLA batteries became a mass-market product ideally suited for both automotive and energy storage applications.
To cope with the new market demands while maintaining or reducing cost, lead-acid battery technology is continuously evolving. Among others, advanced designs have been introduced such as the thin plate VRLA battery (Fig 1 above). This spiral wound VRLA cell design evolution includes sealed inter-cell connectors and one valve per cell, which allow precise internal gas pressure control, thus improving gas recombination efficiency and state-of-charge balance in every cell. Furthermore, the highly compressed cells with microfibre glass separators optimise the specific power and cycle-life.
On the other hand, flat plate VRLA batteries optimise the volumetric energy density by reducing dead space inside the standard prismatic cell design. Additionally, high volume production for automotive application allows significant cost improvements.
Enhanced Flooded Batteries (EFB) (Fig 2) shows how automotive battery designs have incorporated some of the innovations previously developed for spiral wound VRLA batteries like the use of conductive carbon in the negative plate to improve charge acceptance, or the application of glass mat veils in the positive plate to increase cycle-life. New EFB designs also included mechanical devices for high vibration resistance (HVR) and vapour condenser manifolds to reduce water loss at high temperatures.
Battery technology needs to comply with highly demanding requirements like improved charge acceptance to recuperate the energy from short charging events or the ability to maintain the battery performance and cycle-life at partial-state-of-charge. Both VRLA and EFB products have been improved with the use of new materials that, coming from other applications, have been further developed for lead batteries.
Among others, the following innovations have been recently introduced in both automotive and industrial applications:
• Carbon nanomaterials: Carbon Nanotubes (CNTs) improve charge acceptance and cycle-life of the negative plate, even at very low dosage (thanks to its high conductivity and large aspect ratio length/diameter). Although not fully stable in the positive plate, CNTs modify the microstructure of the grid/active material interphase, improving initial performance and corrosion resistance.
• New current collectors: To reduce lead sulfation, carbon felts can be used as current collectors for the negative electrode, thus increasing and maintaining charge acceptance at partial-state-of-charge. There are other materials in development to replace lead positive grids, with a limited success up to now due the highly corrosive environment of the positive electrode. In this regard, the initial laboratory results obtained with titanium-coated mesh (with SnO2 and PbO2) are quite promising.
• Hybrid capacitors: This innovative design, originally consisting of twin carbon+lead negatives and standard PbO2 positive plates, has more recently evolved to a double layer electrode, where the external surfaces of the lead negative plate are covered with two capacity layers.
• Bipolar plates: This technology is still in development, but with significantly improved designs that, using new materials (like polymer/lead foils or silicon with metal coatings), have the potential to eliminate the top lead connectors, thus reducing weight and improving energy density.
Despite the recent improvements, lead batteries are facing strong competition from lithium-ion technologies in the booming markets of e‑mobility and energy storage. The ability of the industry to adapt to the new market requirements with incremental future innovations is key to the long-term survival of lead batteries. In this regard, the following challenges should be addressed to keep a significant role in future:
• Dynamic charge acceptance: Fig 3 shows how charge acceptance is mainly related to the negative active material (NAM) surface area. Organic expanders interact with carbon by reducing the electro-chemically active electrode area. In general, carbon dosage increases whereas the expander amount reduces charge acceptance. New carbon nanomaterials at low dosages may increase charge acceptance while improving initial performance and water loss.
• High temperature endurance: High surface area carbon additives increase water loss under the standard EN 50342 overcharge test conditions (60°C). However, most recent data show that this is not always the case in simulated highway hot environmental conditions (75°C). The correlation between these two different tests is not straightforward for batteries especially designed to improve charge acceptance (EFB+C). Carbon surface electro-chemistry and cell overcharge control are probably the keys to reducing water loss at high temperature.
• Deep-cycle life: The addition of phosphoric acid to the electrolyte improves deep-cycle life but reduce the initial performance. High surface area carbons reduce the over-voltage of the negative, enhancing the recharge ability of the positive electrode. Fig 4 shows that the combination of carbon and phosphoric acid significantly increase deep-cycle life of gel blocks. New additives in the electrolyte and highly compressed VRLA designs may further improve deep-cycle life while improving initial performance.
Lead batteries have been up to know the preferred technology for automotive and industrial use due to its low cost and availability of raw materials. However, lithium-ion is challenging its dominant position due to the cost reduction achieved with high volume production for electric vehicle applications.
The ability of the industry to adapt to the new automotive market requirements with incremental future innovations is key to the long-term survival of lead batteries. On the other hand, significant improvements on performance and total cost of ownership (TCO) are needed to compete with lithium-ion in the industrial markets.
Increasing both fast charge ability and cycle-life are the key to retain some important markets (Energy Storage, Motive Power, Telecoms, UPS) that are now at risk. Finally, focusing on its well-known advantages (safety and sustainability) and developing more environmentally friendly recycling processes will also help to compete with other advanced technologies.